From 49577f8f4555c6e1f140958b2f6caf714266aa8f Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Sat, 26 Sep 2026 14:24:16 -0400
Subject: [PATCH] GVN transmutes of Immediate::Uninit to Immediate::Uninit

---
 compiler/rustc_mir_transform/src/gvn.rs       |  3 ++
 tests/mir-opt/const_prop/maybe_uninit.rs      |  7 ++++
 ...maybe_uninit.u8_array_assume_init.GVN.diff | 36 +++++++++++++++++++
 3 files changed, 46 insertions(+)
 create mode 100644 tests/mir-opt/const_prop/maybe_uninit.u8_array_assume_init.GVN.diff

diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 81a41be1c62da..b5fa022a527e5 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -725,6 +725,9 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
                 }
                 CastKind::Transmute | CastKind::Subtype => {
                     let value = self.eval_to_const(value)?;
+                    if value.is_immediate_uninit() {
+                        return Some(ImmTy::uninit(ty).into());
+                    }
                     // `offset` for immediates generally only supports projections that match the
                     // type of the immediate. However, as a HACK, we exploit that it can also do
                     // limited transmutes: it only works between types with the same layout, and
diff --git a/tests/mir-opt/const_prop/maybe_uninit.rs b/tests/mir-opt/const_prop/maybe_uninit.rs
index 649fea6252216..89c395a7be5bb 100644
--- a/tests/mir-opt/const_prop/maybe_uninit.rs
+++ b/tests/mir-opt/const_prop/maybe_uninit.rs
@@ -8,3 +8,10 @@ pub fn u8_array() -> [MaybeUninit<u8>; 8] {
     // CHECK: _0 = const <uninit>;
     [MaybeUninit::uninit(); 8]
 }
+
+// EMIT_MIR maybe_uninit.u8_array_assume_init.GVN.diff
+pub fn u8_array_assume_init() -> [MaybeUninit<u8>; 8] {
+    // CHECK: fn u8_array_assume_init(
+    // CHECK: _0 = const <uninit>;
+    unsafe { MaybeUninit::uninit().assume_init() }
+}
diff --git a/tests/mir-opt/const_prop/maybe_uninit.u8_array_assume_init.GVN.diff b/tests/mir-opt/const_prop/maybe_uninit.u8_array_assume_init.GVN.diff
new file mode 100644
index 0000000000000..e0c4af329fadc
--- /dev/null
+++ b/tests/mir-opt/const_prop/maybe_uninit.u8_array_assume_init.GVN.diff
@@ -0,0 +1,36 @@
+- // MIR for `u8_array_assume_init` before GVN
++ // MIR for `u8_array_assume_init` after GVN
+  
+  fn u8_array_assume_init() -> [MaybeUninit<u8>; 8] {
+      let mut _0: [std::mem::MaybeUninit<u8>; 8];
+      let mut _1: std::mem::MaybeUninit<[std::mem::MaybeUninit<u8>; 8]>;
+      scope 1 (inlined MaybeUninit::<[MaybeUninit<u8>; 8]>::uninit) {
+      }
+      scope 2 (inlined #[track_caller] MaybeUninit::<[MaybeUninit<u8>; 8]>::assume_init) {
+          scope 3 (inlined transmute_neo::<MaybeUninit<[MaybeUninit<u8>; 8]>, [MaybeUninit<u8>; 8]>) {
+          }
+      }
+  
+      bb0: {
+          StorageLive(_1);
+-         _1 = MaybeUninit::<[MaybeUninit<u8>; 8]> { uninit: const () };
++         _1 = const <uninit>;
+          goto -> bb1;
+      }
+  
+      bb1: {
+-         _0 = copy _1 as [std::mem::MaybeUninit<u8>; 8] (Transmute);
++         _0 = const <uninit>;
+          StorageDead(_1);
+          return;
+      }
++ }
++ 
++ ALLOC0 (size: 8, align: 1) {
++     __ __ __ __ __ __ __ __                         │ ░░░░░░░░
++ }
++ 
++ ALLOC1 (size: 8, align: 1) {
++     __ __ __ __ __ __ __ __                         │ ░░░░░░░░
+  }
+